The present invention relates to producing adherent and oxidation resistant coatings on superalloys, and more particularly, to aluminide coatings having a spallation-resistant alumina layer therein.
High performance superalloys, such as nickel- or cobalt-based superalloys, are being increasingly employed in various types of gas turbine engines used, for example, in the propulsion and power generation industries. High pressure components (e.g., airfoils) of high performance and high fuel efficiency turbine engines generally have a hollow core made of a nickel-or cobalt-based superalloy. These components are subjected to corrosive exhaust gases at extremely high temperatures of up to 1150xc2x0 C. during the operation of a gas turbine engine. As a result, these components are prone to oxidation damage. Various solutions have been tried to prevent such oxidation damage.
One means typically employed is to provide the components with an environmentally-resistant coating, often provided by an aluminum-rich alloy, such as aluminide, whose surface oxidizes to form an aluminum oxide (alumina) scale at elevated temperatures. Such a scale provides a tough, adherent layer that is highly resistant to oxidation and corrosion attack. However, one of the major problems faced by these environmentally-resistant coatings is the spallation of the alumina layer thus formed. Upon subsequent cooling of the turbine blade, the thermal mismatch in the coefficients of thermal expansion of the underlying superalloy substrate and the alumina layer disposed thereon leads to an enormous compressive stress on the alumina layer. Thus, when the alumina layer is repeatedly subjected to such intense thermal cycles, spallation of the alumina layer frequently occurs. This in turn triggers regeneration of alumina when the underlying exposed aluminide layer is oxidized by the air at high temperatures produced during the normal working cycle of a gas turbine engine. This regeneration process depletes available aluminum from the underlying aluminide layer disposed on the surface of the substrate, ultimately limiting the life of the coated component. Furthermore, when a thermal barrier coating is applied on the alumina layer, any spallation of the underlying alumina layer results in loss of the overlying thermal barrier coating, with grave consequences. Therefore, creating a spallation-resistant alumina layer is critical.
One example of such a spallation-resistant alumina interfacial layer is disclosed in U.S. Pat. No. 4,880,614, where a high-purity alumina interfacial layer is provided between the metallic substrate and the ceramic thermal barrier coating.
One embodiment of the present invention embraces an article, comprising:
(a) a superalloy substrate;
(b) an adherent metal aluminide layer disposed on the surface of the substrate; and
(c) an aluminum oxide layer disposed on the surface of the metal aluminide layer,
wherein the aluminum oxide layer includes tensile cracking.
The aluminum oxide layer serves as a spallation-resistant protective layer, greatly enhanced by the tensile cracks contained therein, as discussed below. The layer is substantially free of spallation cracks. The article is often in the form of a turbine engine component, e.g., a high-pressure component such as an airfoil. The article may further include a thermal barrier layer disposed over the aluminum oxide layer. One example is a layer formed of yttria-stabilized zirconia.
The present invention is also directed to a method of producing a spallation-resistant protective layer on the surface of a superalloy substrate, comprising the steps of:
depositing an adherent metal aluminide layer on said substrate,
depositing an aluminum oxide layer on the surface of said metal aluminide layer; and
heating said aluminum oxide layer at an elevated temperature, as discussed below.
Other details regarding the various features of this invention are found in the remainder of the specification.